This invention relates to unflooded bipolar batteries of the immobilized-electrolyte type, and more particularly, to such batteries where several unsealed cells are stacked together in an inert gas filled housing.
Bipolar batteries offer the advantages of high specific power and high specific energy due to their low internal electrical resistance and uniform current distribution. Bipolar batteries, however, are particularly vulnerable to unwanted, wasteful side reactions occurring between adjoining cells resulting from the formation of a continuous film (i.e., only a few micrometers thick) of electrolyte over the external surface of the cell stack. Unless protective measures are taken, such a film can form an ionic bridge between the electrodes of opposite polarity in adjacent cells in the stack and thereby promote unwanted side reactions between those electrodes. Such reactions nonusefully consume the batteries' active materials and drastically shorten the useful life of the battery.
Some have sought to avoid these unwanted intercell reactions by providing insulating seals around the periphery of each cell to ionically isolate the reactive innards thereof from the cell bridging film. Others have encircled the reactive innards with a material which is not wetted by electrolyte, and hence, disrupts the continuity of the film. Such insulating seals and/or nonwettable film disrupters add undesirable weight and complexity to the battery. Moreover, such seals and disrupters are susceptible to deleterious chemical and physical attack. Their ability to withstand such attack materially affects the life and reliability of the batteries in which they are used and once their integrity is broken, the batteries' capcity (i.e., amp-hrs) is rapidly lost. BN, for example, has been used as a seal/film disrupter material in high temperature (i.e., 425.degree. C.), Li bipolar batteries (i.e., Li-Se). The BN not only adds weight to the cell but has been found to be difficult to bond to other components of the battery structure and slowly loses its nonwetting properties with time.
More recently, efforts have been made to reduce the size and weight of the intercell insulating seals. These efforts, however, have been at the expense of increased internal electrical resistance due to constricted current paths and nonuniform current distributions throughout the cell stack.
It is an object of the present invention to eliminate insulating seals and film disrupters in bipolar batteries without comprising internal electrical resistance or current distribution while at the same time maintaining high levels of capacity throughout the useful life of the battery. This and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.